Fuel cells are electrochemical devices that are being developed for automotive and stationary electric power generation. One fuel cell design uses a solid polymer electrolyte (SPE) membrane or proton exchange membrane (PEM), to provide ion transport between a fuel supply electrode and an oxygen electrode of each cell. Gaseous and liquid fuels capable of providing protons are used. Examples include hydrogen and methanol, with hydrogen being favored. Hydrogen is supplied to the fuel cell's anode. Oxygen (as air) is the cell oxidant and is supplied to the cell's cathode. In present designs the electrodes are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote ionization of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. Conductor plates carry away the electrons formed at the anode.
Currently, state of the art PEM fuel cells utilize a membrane made of one or more perfluorinated ionomers such as DuPont's Nafion®. The ionomer carries pendant ionizable groups (e.g. sulfonate groups) for transport of protons through the membrane from the anode to the cathode.
A significant problem hindering the large-scale implementation of fuel cell technology is the loss of electrode performance during extended operation, the cycling of power demand during normal automotive vehicle operation as well as vehicle shut-down/start-up cycling. This invention is based in part on the recognition that a considerable part of the performance loss of PEM fuel cells is associated with the degradation of the oxygen reduction electrode catalyst. This degradation is probably caused by a combination of mechanisms that alter the characteristics of the originally prepared catalyst and its support. Likely mechanisms include growth of platinum particles, dissolution of platinum particles, bulk platinum oxide formation, and corrosion of the carbon support material. Indeed, carbon has been found to corrode severely at electrical potentials above 1.2 volts and the addition of platinum particles onto the surface of the carbon increases the corrosion rate of carbon considerably at potentials below 1.2 volts. These processes lead to a loss in active surface area of the platinum catalyst that leads to loss in oxygen electrode performance.
It is desirable to provide a more effective and durable catalyst and catalyst support combination for use in electrodes of fuel cells.
This invention uses practices disclosed in our above-identified, co-pending application U.S. application Ser. No. 12/704,786, filed Feb. 12, 2010.